As is known in the art, monolithic microwave integrated circuits (MMICs) have a wide range of applications. Typically a plurality of active devices (e.g., field effect transistors (FETs)) are formed in a semiconductor substrate structure and the devices interconnected with microwave transmission lines, also formed on the substrate structure, to form, for example, a plurality of interconnected amplifiers. One type of microwave transmission line is coplanar waveguide (CPW) transmission line.
As is also known in the art, for certain high power applications the bottom side of the CPW MMIC is metalized so that a heat sink can be attached as shown in FIG. 1. We have found that this added metalized surface, in conjunction with the topside metal used for the ground plane of the CPW, forms a two-conductor, parallel plate, system that can support waveguide modes that generate feedback around the amplifiers interconnected by the CPW transmission line resulting in unwanted amplifier oscillation. If the resonance frequency associated with this mode falls within the frequency of operation of the amplifier circuit then it may hinder the proper operation of the circuit. Unwanted oscillation associated with this type of moding was experimentally verified as shown in FIG. 2 with an unwanted oscillation at 9.187 GHz (i.e., an inherent resonance without an input signal) and was found to be a limiter for the proper operation of the circuit. In addition to the waveguide mode between the top and bottom metal surface of the MMIC, another type of moding we have found may occur if the MMIC is place in a flip-chip configuration (FIG. 1) on a printed circuit board (PCB). In this case the two-conductor system is formed by the top metal surface of the MMIC and the ground plane on the PCB. This mode also may disrupt the proper circuit operation in the same way described above.
In one embodiment, a monolithic microwave integrated circuit structure is provided: a semiconductor substrate structure; a plurality of active devices formed in a bottom surface portion of the substrate structure; and a microwave transmission line formed on the bottom surface of the substrate structure having an input section, an output section and a interconnecting section electrically connected between the input section and the output section, such interconnecting section electrically interconnecting the active devices. The semiconductor substrate structure has: a first peripheral region disposed on the top surface thereof over the input section; a inner region disposed on the top surface thereof over the interconnecting section; and a second peripheral region disposed on the top surface thereof over the output section. A heat sink is disposed over the top surface of the substrate structure. A metal layer is disposed on the top surface of the substrate structure under the heat sink. The metal layer has an outer periphery terminating at the outer periphery of the heat sink.
In one embodiment, the microwave transmission line is coplanar waveguide transmission line.
In one embodiment, the monolithic microwave integrated circuit structure includes: a printed circuit board having: electrically conductors therein; electrically conductive bumps on an upper surface of the printed circuit board, such bumps being in electrical contact with the transmission line; and electrically conductive vias passing into the printed circuit board between the electrical conductors in the printed circuit board and the conductive bumps.
In one embodiment, a monolithic microwave integrated circuit structure is provided, comprising: a semiconductor substrate structure; a plurality of active devices formed in a bottom surface portion of the substrate structure; a microwave transmission line formed on the bottom surface of the substrate structure having an input section, an output section and a interconnecting section electrically connected between the input section and the output section, such interconnecting section electrically interconnecting the active devices. The semiconductor substrate structure has: a first peripheral region disposed on the top surface thereof over the input section; a inner region disposed on the top surface thereof over the interconnecting section; and a second peripheral region disposed on the top surface thereof over the output section. A thermally conductive heat sink is disposed over a top surface portion of the substrate structure, such heat sink being disposed over the interconnecting section and having an outer periphery thereof terminating at the first peripheral region and the second peripheral region of the top surface of the substrate structure.
In one embodiment, a metal layer is disposed on the top surface of the substrate structure under the heat sink and wherein the metal layer has an outer periphery terminating at the outer periphery of the heat sink.
The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.